Articles made from electrically nonconductive plastic can be metallized by an electroless metallization process or alternatively by a direct electroplating process. In both processes, the article is first cleaned and etched, then treated with a noble metal and finally metallized. The etching is typically undertaken by means of chromosulphuric acid. The etching serves to make the surface of the article receptive to the subsequent metallization, such that the surfaces of the articles are well-wetted with the respective solutions in the subsequent treatment steps and the deposited metal ultimately has sufficiently firm adhesion on the surface.
For etching, the surface of articles, for example made from acrylonitrile-butadiene-styrene copolymer (ABS copolymer), is etched using chromosulphuric acid, so as to form surface microcaverns in which metal is deposited and subsequently adheres there firmly. After the etching, the plastic is activated for the electroless metallization by means of an activator comprising a noble metal, and then metallized electrolessly. Subsequently, a thicker metal layer can also be applied electrolytically. In the case of the direct electroplating process, which does not need an electroless metallization, the etched surface is typically treated with a palladium colloid solution. Subsequently, the surface is contacted with an alkaline solution comprising copper ions complexed with a complexing agent to increase the conductivity. This step leads to the formation of a copper layer and hence to a metal layer on the surface of the article with elevated conductivity. Thereafter, the article can be directly electrolytically metallized (EP 1 054 081 B1). Etching solutions based on chromosulphuric acid, however, are toxic and should therefore be replaced as possible.
The literature describes attempts to replace etching solutions based on chromosulphuric acid with those comprising permanganate salts. The use of permanganates in an alkaline medium for metallization of circuit boards as a carrier of electronic circuits has long been established. Since the hexavalent state (manganate) which arises in the oxidation is water-soluble and has sufficient stability under alkaline conditions, the manganate, similarly to trivalent chromium, can be oxidized electrolytically back to the original oxidizing agent, in this case the permanganate. The document DE 196 11 137 A1 describes the use of the permanganate also for metallization of other plastics as circuit board material. For the metallization of ABS plastics, a solution of alkaline permanganate has been found to be unsuitable since it was not possible in this way to obtain a reliable, sufficient adhesion strength between metal layer and plastic substrate. This adhesion strength is determined in the “peel test”. It should have at least a value of 0.4 N/mm.
EP 1 0010 52 discloses an acidic permanganate solution which is said to be suitable for use in plastic electroplating. The solutions described therein differ in several respects from the present invention, for example because they use very high acid concentrations and very low permanganate concentrations (e.g. 15 M H2SO4 and 0.05 M KMnO4). EP 1 0010 52 does not report the adhesion strengths achievable by this etching treatment. In-house experiments have shown that the adhesion strengths are below a value of 0.4 N/mm. Moreover, the solutions described in EP 1 0010 52 are unstable. A constant quality of the metallization therefore cannot be achieved.
As an alternative to chromosulphuric acid, WO 2009/023628 A2 proposes strongly acidic solutions comprising an alkali metal permanganate salt. The solution contains about 20 g/l alkali metal permanganate salt in 40-85% by weight phosphoric acid. Such solutions form colloidal manganese(IV) species which are difficult to remove. According to WO 2009/023628 A2, the effect of the colloids even after a short time is that coating of adequate quality is no longer possible. To solve the problem, WO 2009/023628 A2 proposes using manganese(VII) sources which do not contain any alkali metal or alkaline earth metal ions. However, the preparation of such manganese(VII) sources is costly and inconvenient. Toxic chromosulphuric acid is therefore still being used for etching treatment of plastics.
In the conventional electroplating of plastic substrates, in which a first metal layer is first deposited without external current, sometimes less than 1 mg/m2 of palladium on the plastic surface is sufficient to start the metal deposition without external current. In direct electroplating, which does not need electroless metallization, at least 30 mg/m2 to 50 mg/m2 of palladium on the plastic surface is required to enable electrolytic metallization. 40 mg/m2 of palladium is generally sufficient for direct electroplating. These minimum amounts of palladium on plastic surfaces have to date been achievable only when the plastic surfaces have been etched with toxic chromosulphuric acid prior to metallization.